Talk about opening a can of worms. This topic has started some of the larger flame wars on the net.
Windows is the world leader in PCs loaded with there software.
Windows has an easy to use user interface which can be learned easily.
Windows has the largest user base of any operating system world wide.
Windows has the most software available for it
Windows has the largest number of developers of any OS
Microsoft has a large technical support department available to support technical issues
Systems administrators are many and can be easily found to support your corporate environment
Unix is very powerful
Unix typically requires less system resources to run the operating system leaving more system resources for the applications which are running on the system.
Many *nix distributions are open source so you have full source code access
Many *nix distributions are free to install and use
Windows is closed source, meaning that you can not view or modify the source code of the operating system.
Because there are so many applications available it can take you a while to find the application that you like the best.
Windows licenses are very expensive to purchase with the Enterprise Edition of Windows Server costing several thousand dollars per server.
There are much fewer *nix admins available for hire.
Top notch *nix admins can be hard to find.
*nix admins are typically more expensive to hire than Windows admins.
Finding support can be more difficult as same companies who distribute the software do not have large support departments.
*nix can be more complex to setup for users who are not familiar with it.
The user interface is not as easy to use.
There is great debate out there about how much having a *nix server saves you. The initial cost is lower as the purchase price is lower or none. However typically *nix admins salaries are higher than Windows admins as the system is more complex.
I’m sure that there is other stuff, but this is what’s come to mind at 2am.